1. Field of the Invention
The present invention is directed to an apparatus and method for measuring low or trace concentration of compounds in a mixture of gases. The apparatus and method are used for separating ions of different mobilities by passing them through an abrupt change or step in electric field magnitude. By using the separation method and apparatus, compounds of interest may be measured with less interference from other components of a gas mixture which reduces or eliminates the need for prior separation of the components of the gas mixture.
2. Description of the Related Art
A large variety of methods and devices have been used to identify or quantify components of gaseous mixture. Gas chromatographs, mass spectrometers, ion mobility spectrometers, photo ionization detectors and many other such devices are currently employed to make these measurements. Applications for these are many and widely varied. They include monitoring of pollutants in air, analysis of gas mixtures in industrial applications, detection of explosives or toxins, locating leaks in piping, identification of unknown compounds, monitoring of chemical reactions and many more. However, the apparatus and method described herein provides yet another method of detecting and measuring ionizable compounds present in a gaseous mixture. It is sensitive enough to measure low or trace concentrations of some compounds and offers advantages over currently available detection technology under some conditions. It may be used as a stand alone detector or in combination with a separation device, such as, but not limited to, gas chromatography.
Over the years, several devices have been developed that use ion mobility to detect or measure compounds in a gaseous mixture. Ion mobility spectrometers (IMS) are one of the most common. Clement et al., “Instrumentation for Trace Organic Monitoring,” (1992), Lewis Publishers, Inc., Chelsey, Mich. and Carr, “Plasma Chromatography,” (1984), Plenum Press, New York, N.Y. gave descriptions of IMS operations. Basically, an IMS collects a time of flight spectrum of ions that have been allowed to drift through a region of constant electric field. One or more ion gates allows a short pulse of ions to enter the drift region and a spectrum is collected as a function of time after the gate is opened. The short gate open times allow the different ions to separate completely and form individual peaks in the spectrum.
Another type of common detector used with gas chromatographs is the Electron Capture Detector or ECD. In this detector, a radioactive element, such as Nickel-63 is used to create free electrons. Voltage is applied to two electrodes in the ECD in short pulses. This causes the free electrons to move to one of the electrodes and creates a detectible current. Pulses are kept short enough that the electrons, which have extremely high mobility, move to the collector, but the slower ions do not have time to reach the electrode. The introduction of compounds that capture electrons into the ECD reduces the number of free electrons and, thus, the current through the ECD is reduced. Although the ECD does use difference in mobilities to separate ions from free electrons, the ECD uses a pulsed electric field.
U.S. Pat. No. 3,522,425 to Rich describes an apparatus and method whereby ions of a particular mobility may be selectively separated from ions of different mobility in a gas stream and their abundance measured. This is accomplished by isolating an ion bearing sample of gas in a conduit through which the gas flows at a predetermined rate and applying a predetermined electric field thereto so that the ions of desirability are retained while ions of different mobilities are removed, then discharging the retained ions and measuring the thus obtained current pulse.
U.S. Pat. No. 4,271,357 describes a device used to detect trace quantities of chemicals in air. It works by using a gas jet to carry ions through a region of opposing electric field. The lower mobility ions are carried through the region while the higher mobility ions move upstream in the jet and eventually wander out of the gas jet. However, each of these prior art disclosures have certain drawbacks which are overcome by the claimed apparatus and method. These and other advantages of the invention will become apparent with reference to the following description.
One of the problems with detection of trace levels of compounds in gases is that the detectors often respond to components of the original gas mixture. For example, halogenated compounds readily form negative ions by capturing free electrons. Their presence can then be detected by allowing them to discharge on an electrode and measuring the resulting current or by measuring the decrease in current from the free electrons. The difficulty with this measurement technique is that oxygen will also capture electrons and form negative ions. When measuring halogenated compounds in air, the oxygen, which makes up 21% of air, typically saturates the detectors so the halogenated compounds can not be detected. This requires the halogenated compounds to be separated from the oxygen prior to being introduced into the detector. However, oxygen has a higher mobility than most halogenated compounds, so the electric fields in the SEFD may be adjusted to trap the halogenated compounds at the screen while allowing the oxygen to continue to move upstream. As can be seen from the following disclosures, the invention is capable of detecting very low concentrations of halogenated compounds in air without any prior separation.